Kind of embarrassed to point out that there are many FOSS advocates here who aren’t up to date on the nuance of all features of proprietary apps. Even for those who are, it is a bit vague for you to request an equivalent to WhatsApp communities without specifying which parts you actually need and why (otherwise we would need to clone the whole system that is surely out of scope). It’s even worse, as you seem to be forwarding a request from a third party, so it would be best if they could chime in. Just noting that you may receive further answers if you elaborated a bit and ideally link to a few websites that explain the concepts that interest you.